The transient hot wire method (THW) is a very popular, accurate and precise technique to measure the thermal conductivity of gases, liquids,[1] solids,[2] nanofluids[3] and refrigerants[4] in a wide temperature and pressure range.
The technique is based on recording the transient temperature rise of a thin vertical metal wire with infinite length when a step voltage is applied to it.
The wire is immersed in a fluid and can act both as an electrical heating element and a resistance thermometer.
An ASTM standard is published for the measurements of engine coolants using a single-transient hot wire method.
[7] 200 years ago scientists were using a crude version of this method to make the first ever thermal conductivity measurements on gases.